Therapsida

Therapsida

Therapsids are an order of synapsids (Class Synapsida). Traditionally, synapsids were referred to as reptiles and were known as the "mammal-like reptiles". However, they are now classified as a sister-group to the reptiles, and are phylogenetically closer to the mammals. Indeed, when the term is used cladistically, the taxon also includes the mammals, which are descended from the cynodont therapsids. All other lineages of the therapsids are extinct; the last known survivors of the non-mammalian therapsids lived in the Early Cretaceous period.

Characteristics

Therapsids' temporal fenestrae were larger than those of the pelycosaurs. The jaws of therapsids were more complex and powerful and the teeth were differentiated into frontal incisors for nipping, large lateral canines for puncturing and tearing, and molars for shearing and chopping food. Therapsids' legs were positioned more vertically beneath their bodies than were the sprawling legs of reptiles and pelycosaurs.

Like all land animals, the therapsids were seriously affected by the Permian–Triassic extinction event, with the very successful gorgonopsians dying out altogether and the remaining groups, dicynodonts, therocephalians and cynodonts of a few species, each surviving into the Triassic. The dicynodonts, now represented by a single family of large stocky herbivores, the Kannemeyeridae, and the medium-sized cynodonts (including both carnivorous and herbivorous forms), flourished worldwide, throughout the Early and Middle Triassic. They died out across much of Pangea at the end of the Carnian (Late Triassic), although they continued for some time longer in the wet equatorial band and the south.
Some exceptions were the still further derived eucynodonts. At least three groups of them survived. They all appeared in the Late Triassic epoch.

Some non-eucynodont cynodonts survived the Permian-Triassic extinction, such as Thrinaxodon but only to become extinct by the Middle Triassic.

The therocephalians, relatives of the cynodonts, managed to survive the Permian-Triassic extinction and continued to diversify through the Early Triassic period. Approaching the end of the epoch, however, the therocephalians were declining to extinction and eventually became extinct, possibly due to climatic changes and competition from cynodonts and other animals struggling to survive.

Paleocene Therapsid?

Chronoperates is a newly described genus of vertebrate that lived 55 million years ago. Its true identity is still debated and it has even been suggested that it is a symmetrodont. Should it turn out to be a therapsid, the extinction date for this group would be pushed forward almost 45 million years.